In various known key telephone systems, the implementation of the many available services or features required additional circuits in each station set and additional conductors running from each station set to the key service unit (KSU). It is not uncommon to have as many as 50 pairs of conductors connecting each station set to the KSU. Consequently, the time, effort and cost of installing, modifying or moving such key telephone sets has become excessive.
Some attempts have been made to overcome these problems by implementing key telephone service features by means of signal multiplexing techniques, thus achieving substantial reductions in the number of conductor pairs required for connection of each key telephone set to the KSU. Although this approach has alleviated the cost and space problems somewhat, it has been only moderately successful. As before, the addition of telephone services or features required circuit modifications in the KSU and in the station sets. Also, such signal multiplexing systems frequently employ a common control circuit to supervise signalling and control for all of the station sets. Thus, failure of the common control circuit causes the entire system to fail.